SUMMARYA comprehensive range of phenylacetic acids substituted with nitro, halogen, methyl, amino, hydroxyl and N‐acetylamino groups have been synthesized and their growth‐regulating activities assessed in the wheat cylinder, pea curvature and pea segment tests. The influence of substituents on molecular shape is shown to be more important in determining activity than their effects on electron distribution.Studies with 2,6‐disubstituted phenylacetic acids have indicated that the most active compounds can attain a certain spatial configuration in which one surface of the molecule, including the plane of the ring system, is flat and the carboxyl group is above with its axis of rotation perpendicular to this surface.Positional requirements for growth‐regulating activity in phenylacetic acids are shown to be less important than in the phenoxyacetic and benzoic acids.

S U M M A R YThe plant growth-regulating activities of all the mono-and di-chlorosubstituted a-hydroxy-phenylacetic (mandelic) acids and their methoxy derivatives have been determined in the wheat coleoptile cylinder, pea segment and pea curvature tests. I n general, chloro-substituted a-hydroxy acids were less active in all three tests than the corresponding a-methoxy acids. The a-methoxy compounds and the dichloromandelic acids were more active in pea than in wheat tissues.These results are discussed in relation to the plant growth-regulating activities of the corresponding phenylacetic and phenoxyacetic acids.